Biogenesis of Organelles (Part 1)
1) A method to obtain component parts
DNA, RNA, protein (need to be transported), lipids, ions, water
Protein Trafficking/ Translocation Routes
Routes, topology (protein shape), multiple sources of protein in endosymbionts ,
signal sequences
When an eukaryotic cell reproduces by dividing in to, it has to duplicate membrane
enclosed organelles – cells cannot make them from scratch, organelles form from
pre-existing ones
Only DNA replication needs a template, other organelles will duplicate (apart from
centrioles – mitochondria and ER cannot be created from scratch
Organelle growth requires a supply of lipids and appropriate proteins
Organelle ‘Source” of proteins
Mitochondria, chloroplasts, Cytosol
interior of nucleus
Golgi apparatus, lysosomes, Delivered indirectly via ER
endosomes, nuclear Proteins enter ER via cytosol, some
membranes are retained, some are transported
by vesicles to Golgi apparatus and
then to other organelles or to plasma
membrane
Synthesis of most proteins begins on cytoplasmic ribosomes except some
mitochondrial/chloroplast proteins (synthesised in organelles)
Fate of any protein molecule synthesised in the cytosol depends on the amino acid
sequence – some contain a sorting signal that directs the protein to the organelle,
those without remain in the cytosol
,Importing a protein from the cytosol/organelle to an
organelle
Selective gates – either facilitated diffusion or active
pump
Proteins moving from cytosol into the nucleus are
transported through nuclear pores that penetrate the
double nuclear membrane: actively transport specific
macromolecules but allow free diffusion of smaller
molecules
Selection + channel
Proteins moving from cytosol into
ER/mitochondria/chloroplasts transported across
organelle membrane by protein translocators located in the membrane – proteins
may have to unfold to pass through or be fully folded
Selection + fusion/fission
Proteins moving from the ER onward and from one compartment of the
endomembrane system to the other – proteins are transported by transport
vesicles, which become loaded with proteins from the lumen/compartment , as they
pinch off from its membrane then discharge cargo via fusion with another
membrane
Two protein trafficking sources in endosymbionts
Three components of protein trafficking
1. Targeting sequences (1 and 2 define specificity)
2. Receptors
3. Membrane translocation machineries (2 and 3 allows import into organelle)
,Signal Sequence Hypothesis (Blobel and
Sabatini, 1971)
GFP protein studies and swapping signal
sequences show the necessity for these
sequences in order for proteins to be in
the correct destination
Typical sorting signal on proteins – 15-
60 amino acids: sequence is often
removed from the finished protein
once it has been sorted
Signal sequences are both necessary
and sufficient to direct a protein to a particular organelle – shown through genetic
engineering (deletion of sequence of transfer of sequence from one protein to
another)
Physical properties of the sequences (eg. Hydrophobicity, acidity) seem more
significant for function than specific amino acid sequence
Typical Targeting Sequences
- Complex binding sites for target receptors targeting sequence dependent on
tertiary protein structure
Target Receptors
Targeting Sequences Receptor 1 (Location) Receptor 2 Mechanism of
(Location) Entry
, Nuclear localisation Importin- α Importin-β Nuclear pore
sequence (cytosol) (cytosol)
ER signal sequence Signal recognition SRP-Receptor Channel
particle (SRP) (ER-membrane)
(cytosol)
Mitochondrial (HSP70 MSF) TOM20/22 Channel
targeting sequence (cytosol) (OMM)
Peroxisomal targeting Pex5p/Pex7p Pex13p/Pex14p Pore
sequence (cytosol) (cytosol)
Lysosomal targeting Mannose-6-phosphate Adaptors n/a
sequence receptor (trans Golgi, Vps10/sortilin
late endosomes
Mitochondrial Import
Signal sequence at the N-terminus
Proteins destined for organelle are translocated
simultaneously across both the inner and outer
membranes at specialised sites where the two
membranes are in contact with each other
Each protein unfolded as it is transported – signal
sequence removed after translocation complete
Chaperone proteins inside organelles help to pull
protein across the two membranes and to refold
protein
Subsequent transport within the organelle requires
further sorting signals in protein, often only
exposed after first signal is removed
Eg. Insertion of transmembrane proteins
into the inner membrane is guided by
signal sequences in the protein that start
and stop the transfer process across the
membrane
Growth and maintenance of mitochondria
and chloroplasts require import of new
proteins and lipids into membranes – most
membrane phosopholipids imported from
the ER
Phospholipids transported individually by water-soluble lipid carrying proteins that
extract a phospholipid molecule from one membrane and deliver to another
1) A method to obtain component parts
DNA, RNA, protein (need to be transported), lipids, ions, water
Protein Trafficking/ Translocation Routes
Routes, topology (protein shape), multiple sources of protein in endosymbionts ,
signal sequences
When an eukaryotic cell reproduces by dividing in to, it has to duplicate membrane
enclosed organelles – cells cannot make them from scratch, organelles form from
pre-existing ones
Only DNA replication needs a template, other organelles will duplicate (apart from
centrioles – mitochondria and ER cannot be created from scratch
Organelle growth requires a supply of lipids and appropriate proteins
Organelle ‘Source” of proteins
Mitochondria, chloroplasts, Cytosol
interior of nucleus
Golgi apparatus, lysosomes, Delivered indirectly via ER
endosomes, nuclear Proteins enter ER via cytosol, some
membranes are retained, some are transported
by vesicles to Golgi apparatus and
then to other organelles or to plasma
membrane
Synthesis of most proteins begins on cytoplasmic ribosomes except some
mitochondrial/chloroplast proteins (synthesised in organelles)
Fate of any protein molecule synthesised in the cytosol depends on the amino acid
sequence – some contain a sorting signal that directs the protein to the organelle,
those without remain in the cytosol
,Importing a protein from the cytosol/organelle to an
organelle
Selective gates – either facilitated diffusion or active
pump
Proteins moving from cytosol into the nucleus are
transported through nuclear pores that penetrate the
double nuclear membrane: actively transport specific
macromolecules but allow free diffusion of smaller
molecules
Selection + channel
Proteins moving from cytosol into
ER/mitochondria/chloroplasts transported across
organelle membrane by protein translocators located in the membrane – proteins
may have to unfold to pass through or be fully folded
Selection + fusion/fission
Proteins moving from the ER onward and from one compartment of the
endomembrane system to the other – proteins are transported by transport
vesicles, which become loaded with proteins from the lumen/compartment , as they
pinch off from its membrane then discharge cargo via fusion with another
membrane
Two protein trafficking sources in endosymbionts
Three components of protein trafficking
1. Targeting sequences (1 and 2 define specificity)
2. Receptors
3. Membrane translocation machineries (2 and 3 allows import into organelle)
,Signal Sequence Hypothesis (Blobel and
Sabatini, 1971)
GFP protein studies and swapping signal
sequences show the necessity for these
sequences in order for proteins to be in
the correct destination
Typical sorting signal on proteins – 15-
60 amino acids: sequence is often
removed from the finished protein
once it has been sorted
Signal sequences are both necessary
and sufficient to direct a protein to a particular organelle – shown through genetic
engineering (deletion of sequence of transfer of sequence from one protein to
another)
Physical properties of the sequences (eg. Hydrophobicity, acidity) seem more
significant for function than specific amino acid sequence
Typical Targeting Sequences
- Complex binding sites for target receptors targeting sequence dependent on
tertiary protein structure
Target Receptors
Targeting Sequences Receptor 1 (Location) Receptor 2 Mechanism of
(Location) Entry
, Nuclear localisation Importin- α Importin-β Nuclear pore
sequence (cytosol) (cytosol)
ER signal sequence Signal recognition SRP-Receptor Channel
particle (SRP) (ER-membrane)
(cytosol)
Mitochondrial (HSP70 MSF) TOM20/22 Channel
targeting sequence (cytosol) (OMM)
Peroxisomal targeting Pex5p/Pex7p Pex13p/Pex14p Pore
sequence (cytosol) (cytosol)
Lysosomal targeting Mannose-6-phosphate Adaptors n/a
sequence receptor (trans Golgi, Vps10/sortilin
late endosomes
Mitochondrial Import
Signal sequence at the N-terminus
Proteins destined for organelle are translocated
simultaneously across both the inner and outer
membranes at specialised sites where the two
membranes are in contact with each other
Each protein unfolded as it is transported – signal
sequence removed after translocation complete
Chaperone proteins inside organelles help to pull
protein across the two membranes and to refold
protein
Subsequent transport within the organelle requires
further sorting signals in protein, often only
exposed after first signal is removed
Eg. Insertion of transmembrane proteins
into the inner membrane is guided by
signal sequences in the protein that start
and stop the transfer process across the
membrane
Growth and maintenance of mitochondria
and chloroplasts require import of new
proteins and lipids into membranes – most
membrane phosopholipids imported from
the ER
Phospholipids transported individually by water-soluble lipid carrying proteins that
extract a phospholipid molecule from one membrane and deliver to another
